Linear accelerator with x-ray absorbing insulators

ABSTRACT

Annular insulators for supporting successive annular electrodes in a linear accelerator have embedded X-ray absorbing shield structures extending around the accelerating path, the shield members disposed to intercept X-ray radiation without disrupting the insulative effect of the insulator members. In preferred forms the structure comprises a plurality of annular members of heavy metal disposed in an X-ray blocking array, spaced from each other by the insulating substance of the insulator member.

United States Patent Rose Sept. 2, 1975 [54] LINEAR ACCELERATOR WITH X-RAY 3,675,061 7/1972 Harrison 313/359 3,740,554 6/1973 Morgan 250/423 ABSORBING INSULATORS Inventor:

[73] Assignee:

Mass.

[22] Filed:

Feb. 19, 1974 Appl. No.: 443,718

Peter H. Rose, Bedford, Mass.

Extrion Corporation, Gloucester,

US. Cl. 250/508; 250/514; 313/360 Int. Cl. HOlU 35/16 Field of Search 250/423, 508, 510, 514;

References Cited UNITED STATES PATENTS Wilson 313/360 Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms [5 7] ABSTRACT Annular insulators for supporting successive annular electrodes in a linear accelerator have embedded X-ray absorbing shield structures extending around the accelerating path, the shield members disposed to intercept X-ray radiation without disrupting the insulative effect of the insulator members. In preferred forms the structure comprises a plurality of annular members of heavy metal disposed in an X-ray blocking array, spaced from each other by the insulating substance of the insulator member.

10 Claims, 5 Drawing Figures SHE? 1 [1F 2 FIG I FIG 20 LINEAR ACCELERATOR WITH X-RAY ABSORBING INSULATORS Numerous research and industrial problems can be solved with the aid of beams of high-energy particles (an example being the implantation of impurities in semiconductor wafers to produce n-type or p-type regions within the material). One mechanism for producing such beams is the linear accelerator in which charged particles are accelerated within an evacuated chamber by applying a voltage gradient to them. The voltage gradient is generated by a set of metal electrodes, with successively increasing or decreasing voltages imposed on them, the direction of the gradient depending upon the sign of the charge of the particles being accelerated. These electrodes are separated by insulating members made of ceramic, epoxy, plastics or similar materials which provide structural rigidity and which enclose the evacuated space within the chamber without permitting electrical breakdown between electrodes.

One problem which arises is that of shielding against X-rays. For instance during the acceleration of ions, secondary electrons are produced by collisions between the accelerated ions and residual gas molecules and are accelerated by the voltage gradient in the opposite sense to that of the positive ions. The secondary electrons collide with the beam control slit structure found at the entrance to the acceleration chamber and produce X-rays which disperse in all directions. It is important to shield personnel working nearby from the effects of the X-rays. X-rays may escape by paths leading through the acceleration chamber itself and in particular by paths passing through the insulating members, since insulating materials have low atomic weights (i.e., low Z-materials) and X-ray absorption is small in such materials.

According to the present invention it is realized that external shielding about the accelerator is unnecessarily expensive and cumbersome and that compact shielding against X-rays escaping through the accelera tion chamber is obtainable by embedding X-ray absorbing shield structure in the insulator members themselves, preferably in the form of a plurality of annular heavy metal members in an array with insulating material between them. This arrangement permits insulation against high-voltage electrical breakdown between accelerating electrodes and simultaneously provides shielding against X-rays escaping from the chamber.

Further objects and features of the invention will be understood from the following description of preferred embodiments taken in conjunction with the drawings wherein:

FIG. I is a diagrammatic view of a linear accelerator used to implant ions in a semiconductor wafer;

FIG. 2 is a cross-sectional view of the acceleration tube of FIG. 1 showing details of the accelerating electrodes and the annular insulating members according to a preferred embodiment of the invention;

FIG. 2a is an exploded view partially in cross section of a portion of the structure of FIG. 2;

FIGS. 3 and 4 are cross-sectional views of other preferred embodiments of the invention.

With reference to FIG. 1, charged particles of the type desired are generated in ion source 2, powered by KV power supply 3, accelerated initially through electrodes 4, powered by l KV power supply 5, and ana high energy by a voltage gradient imposed on acceleration tube 10 by high voltagepower supply 12. The accelerated beam of particles passes through focussing and deflecting structure 14, with substructures such as focussing lens 14a, neutral beam trap and beam gate deflecting plates 14b, Y-axis scanner 14c, beam trap and gate plate 14d and X-axis scanner Me, which produces a well-defined beam of desired particles and enables scanning of the beam across the target wafer 16. As denoted by the dashed lines the entire beam line is surrounded by a chamber which is evacuated to reduce scattering of the beam particles from gas molecules within the system. i.

FIG. 2 illustrates the features of a multi-electrode acceleration tube. The beam ofparticles moves from left to right down the axis 18 of the acceleration tube. End plates 20 provide mounting flanges for connecting the acceleration tube to the remainder of the vacuum chamber. O-ring grooves 22 are cut into the faces of end plates 20 and O-rings (not shown) are mounted therein to provide vacuum seals when the system is assembled. Accelerating electrode rings 24 are spaced along theaxis 18 of the acceleration tube and are separated from one another, by insulating members 26, made of epoxy in this embodiment. Partial shielding of the X-rays generated within the acceleration chamber is accomplished by the use of various shielding rings 28, mounted on electrode rings 24. Equipotential rings 30 are mounted on the outer periphery of all electrode rings 24 to prevent electrical breakdown and to facilitate application of the voltage to the electrode rings.

In FIGS. 2 and 2a it can be seen that insulating members 26 are of generally toroidal or annular shape with an approximately cylindrical inner surface 32 and a bowed outer surface 34. Outer surface 34 is exposed to the atmosphere and it must therefore be shaped to ensure a long potential breakdown path compared to that on the inner surface which is exposed to vacuum. Embeddedin the epoxy of the insulating members 26 are annular heavy metal shield members 36, of lead in this embodiment. As suggested by the .path lines L of FIG. 2, the metal members 36 are disposed in an array which intercepts all straight line paths originating in that portion of beam control slit 8 which may be struck by the accelerated secondary electrons. Straight line paths from the inside to the outside of the acceleration chamber exist, e.g., path L, but such paths originate in areas of the acceleration chamber where X-ray production is negligible. Partial shielding rings 28 aid in the shielding, but are not able to completely shield the X-rays because of their low density. The metal members 36 are also placed to prevent electrical breakdown between electrode rings 24, there being a substantial thickness of insulating material between each ring and the next,

and between each ring and at least one of the electrodes. Also the rings are provided with rounded surfaces in regions R of any possible breakdown to minimize that possibility.

Typical dimensions (as seen in FIG. 2) of the acceleration tube are:

a l.5 inches e .25 inch b 1.0 inches f= 9.50 inches c 2.125 inches g l0 inches d .125 inch The electrodes in this embodiment are made of aluminum; the insulating material is high-voltage insulating epoxy and a typical voltage differential between successive electrodes is 25 Kilovolts.

FIG. 3 illustrates a second preferred embodiment of the invention. In this embodiment the possible straight line paths from the inside to the outside of the acceleration chamber are fewer than those in FIG. 2. The solid angle subtended by such paths is close to or equal to zero. Primed numbers correspond to equivalent structures in FIG. 2 which have been modified. Both the embodiments of FIG. 2 and FIG. 3 are particularly suitable for casting of the insulating members, since the lead metal members can be supported from the sides during casting.

In the embodiment of FIG. 4 all straight line paths through the wall of the insulating member are blocked by the thickness of one of the annular members.

It will be apparent from the above description that various insulating materials such as ceramic, epoxy, glass, and so on may be used and that various materials of high atomic weight may be used as shielding members.

It will be understood that numerous other variations in the specific construction are possible within the spirit and scope of the following claims:

What is claimed is:

1. In an accelerator comprising a multiplicity of accelerating electrodes of annular configuration axially aligned to define an accelerating path for charged particles and annular insulator members disposed between adjacent electrodes, supporting said electrodes and enabling a voltage difference to be imposed between adjacent electrodes, the improvement wherein said insulator members include discrete, annular members of radiation absorbing material embedded within said insulator members and extending around said accelerating path, said annular members spaced from internal and external surfaces of said insulator members by insulating material, said annular members selectively disposed to intercept X-ray radiation along possible exit paths from the internal portions of said accelerator without disrupting the insulative effect of the respective insulator members, and without providing a conductive path between said annular electrodes.

2. The apparatus of claim 1 wherein a plurality of said annular members is embedded within a single said insulator member, said annular members disposed therein in X-ray blocking array, the annular absorbing members being spaced from each other by insulating substance of the insulator member and being composed of elemental heavy metal.

3. The apparatus of claim 2 wherein at least one of said annular absorbing members extends to a side of said annular insulator that abuts one of said adjacent electrodes.

4. The apparatus of claim 3 wherein another said annular absorbing member extends to the side of said insulator member which abuts the other said adjacent electrode, annular absorbing members in said insulating member arranged so that no straight line can be projected between said sides of said insulator member which connects the region internal of said insulator member with the external environment without being intercepted by one of said annular absorbing members.

5. The improvement of claim 2 wherein said annular absorbing members have rounded surfaces in regions of potential difference high enough to cause possible electrical breakdown.

6. The improvement of claim 1 wherein a said embedded annular absorbing member comprises a casting.

7. An insulator member for an accelerator having a multiplicity of successive annular accelerating electrodes, said insulator member being adapted to lie between adjacent electrodes, and comprising an annular body of insulator material having embedded therein an X-ray absorbing shield structure in the form of a plurality of annular heavy metal X-ray absorbing members disposed in an X-ray blocking array adapted to prevent X- rays from escaping from said accelerator into the environment, the annular heavy metal members being spaced from each other by insulating material.

8. The apparatus of claim 7 wherein at least one of said annular absorbing members extends to a side of said annular insulator that abuts a said electrode.

9. The apparatus of claim 7 wherein another said annular absorbing member extends to the opposite side of said insulator member which abuts the next successive electrode, annular absorbing members in said insulating member arranged so that no straight line can be projected between said sides of said insulator member which connects the region internal of said insulator member with the external environment without being intercepted by one of said annular absorbing members.

10. The improvement of claim 7 wherein said annular absorbing members have rounded surfaces in regions of potential difference high enough to cause possible electrical breakdown.

l= I l 

1. In an accelerator comprising a multiplicity of accelerating electrodes of annular configuration axially aligned to define an accelerating path for charged particles and annular insulator members disposed between adjacent electrodes, supporting said electrodes and enabling a voltage difference to be imposed between adjacent electrodes, the improvement wherein said insulator members include discrete, annular members of radiation absorbing material embedded within said insulator members and extending around said accelerating path, said annular members spaced from internal and external surfaces of said insulator members by insulating material, said annular members selectively disposed to intercept X-ray radiation along possible exit paths from the internal portions of said accelerator without disrupting the insulative effect of the respective insulator members, and without providing a conductive path between said annular electrodes.
 2. The apparatus of claim 1 wherein a plurality of said annular members is embedded within a single said insulator member, said annular members disposed therein in X-ray blocking array, the annular absorbing members being spaced from each other by insulating substance of the insulator member and being composed of elemental heavy metal.
 3. The apparatus of claim 2 wherein at least one of said annular absorbing members extends to a side of said annular insulator that abuts one of said adjacent electrodes.
 4. The apparatus of claim 3 wherein another said annular absorbing member extends to the side of said insulator member which abuts the other said adjacent electrode, annular absorbing members in said insulating member arranged so that no straight line can be projected between said sides of said insulator member which connects the region internal of said insulator member with the external environment without being intercepted by one of said annular absorbing members.
 5. The improvement of claim 2 wherein said annular absorbing members have rounded surfaces in regions of potential difference high enough to cause possible electrical breakdown.
 6. The improvement of claim 1 wherein a said embedded annular absorbing member comprises a casting.
 7. An insulator member for an accelerator having a multiplicity of successive annular accelerating electrodes, said insulator member being adapted to lie between adjacent electrodes, and comprising an annular body of insulator material having embedded therein an X-ray absorbing shield structure in the form of a plurality of annular heavy metal X-ray absorbing members disposed in an X-ray blocking array adapted to prevent X-rays from escaping from said accelerator into the environment, the annular heavy metal members being spaced from each other by insulating material.
 8. The apparatus of claim 7 wherein at least one of saId annular absorbing members extends to a side of said annular insulator that abuts a said electrode.
 9. The apparatus of claim 7 wherein another said annular absorbing member extends to the opposite side of said insulator member which abuts the next successive electrode, annular absorbing members in said insulating member arranged so that no straight line can be projected between said sides of said insulator member which connects the region internal of said insulator member with the external environment without being intercepted by one of said annular absorbing members.
 10. The improvement of claim 7 wherein said annular absorbing members have rounded surfaces in regions of potential difference high enough to cause possible electrical breakdown. 